Gene Discovery- Immunotherapy has been most successful when targeted to differentiation antigens expressed on hematopoietic tumors in large part because the target antigens are not expressed on essential cells or organs. Other cancers in which this approach could be successful are prostate, breast and ovary. To identify target antigens in prostate and breast cancers we have developed a computer based method to search the EST data base for ESTs that are expressed in prostate or breast cancer and not in other essential organs or cells. We have validated this approach experimentally and used it to identify several genes/proteins expressed in prostate cancer and not in essential normal organs. These include TARP, PAGE4, and GDEP. TARP is a mitochondrial protein that is also expressed in breast cancer. In the past year we have identified a new gene NGEP expressed in prostate and prostate cancer, encoding a membrane protein with seven membrane-spanning domains. We also identified POTE, a large gene family, expressed in prostate, ovary, testis, placenta and many cancers. We have prepared polyclonal and monoclonal antibodies to both these proteins and are using them to determine their cellular location and tissue expression patterns. We have also identified a new gene highly expressed in breast cancers (BPSR) and have produced antibodies to determine in which cells it is expressed. Preliminary results show very high expression in most breast cancers.